Project Summary/Abstract Low-grade astrocytomas are the most common brain tumor in children. Standard of care therapies have limited efficacy and treatment-related morbidity is significant. The broad objective of our study plan is to develop potent, brain-penetrant, targeted therapeutics for pediatric low-grade astrocytoma (PLGA). Towards this end, mutated, constitutively active forms of the BRAF protein kinase are expressed in ~75% of all PLGAs and are attractive targets for drug development. A minor cohort of PLGAs express V600E BRAF - a point mutation oncoprotein that is a frequent driver of malignant melanoma in adult patients. More commonly, PLGAs express a truncation/fusion oncoprotein known as KIAA1549:BRAF. Small molecule type 1 RAF inhibitors developed for adult melanoma have poor brain penetrance and are, moreover, ineffective antagonists of KIAA1549:BRAF. Against this backdrop, we have three specific aims: Aim 1 is to examine the clinical activity of TAK-580 in progressive, BRAF-mutant PLGAs. Under auspices of this SPORE, we showed that TAK-580 (a clinical stage type 2 RAF inhibitor) has good brain penetrance and targets both forms of the BRAF oncoprotein. A phase 0/I/II trial of TAK-580 in children with BRAF mutant low-grade gliomas tumors has been initiated. Using clinical materials from the phase I and II components of the trial we will establish the pharmacokinetics and pharmacodynamics of TAK-580 in children relative to adult patients where the drug has been previously evaluated. In the Phase 0 component of this trial, we will directly measure drug penetration into tumors. Aim 2 is to define the impact of cellular and genetic modifiers on response of PLGAs to TAK-580. An ?inconvenient truth? in precision medicine is that target expression does not guarantee responsiveness to a targeted therapeutic. For example, type 1 RAF antagonists are effective inhibitors of V600E BRAF in melanoma but are ineffective on the same oncoprotein in colon cancers. Accordingly, as the TAK-580 clinical trial goes forward, we will conduct a series of in vitro ?avatar? trials on primary patient tumor cells grown in a synthetic hydrogel system developed in collaboration with a bioengineering group at MIT. This system is similar to ?organoid? systems developed for other solid tumors. Aim 3 is to develop second generation brain-penetrant drugs for BRAF-mutant PLGA with enhanced selectivity for KIAA1549:BRAF. TAK-580 targets both forms of the BRAF oncoprotein, but WT BRAF is also inhibited by the drug. Thus, TAK-580 is a ?signal transduction inhibitor? but not a true targeted therapeutic. Although signal transduction inhibitors can be highly efficacious cancer medicines (e.g., imatinib or trastuzumab), a drug that is truly mutant-specific would be preferable for growing children. By far the most common form of BRAF oncoproteins in PLGA is a truncation/fusion protein known as KIAA1549:BRAF. In this aim, we take a mechanism-based approach to development of a drug that selectively targets KIAA1549:BRAF.